INTRODUCTION

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Nocardia asteroides is increasingly recognized as a serious cause of pulmonary disease in both normal and immunocompromised humans (3). A murine model for the investigation of host responses during pulmonary nocardiosis has been developed (6). Cells of N. asteroides GUH-2, during the log phase of growth, adhere to and invade the bronchiolar epithelium of mice following intranasal inoculation (2, 4). At the same time, some of these bacteria reach the alveoli, where they are phagocytized by alveolar macrophages (8, 9). Within hours, the nocardiae initiate growth, which induces an extensive inflammatory response characterized by infiltration with polymorphonuclear phagocytes (PMNs) resulting in bronchopulmonary pneumonia (3, 5, 6). The progressive nature of this pulmonary response is dependent on the relative inoculum size, which dictates the extent of pulmonary damage and host responsiveness (5, 6, 7). For example, with a lethal dose (10⁷ CFU/lung), the mice become acutely ill and die within 3 to 7 days. However, following a lower, nonlethal inoculum, the mice develop symptoms within 24 h that may worsen over the next 24 to 48 h; but at 5 days postinfection, the mice appear improved. Seven days after inoculation, these mice appear to be fully recovered (14, 19).

Histologically, the pneumonia reaches its peak during the first 72 h, but after 5 days the pulmonary infiltrates become stabilized and begin to resolve, so that by 7 days postinfection relatively few inflammatory infiltrates can be found. In general, the structural integrity of the alveoli appears intact and relatively normal (6, 14, 19). These observations suggest that a critical host response occurs within the lungs between 3 and 5 days after intranasal administration of log-phase cells of N. asteroides. During this time period, nocardial growth is terminated, resulting in complete resolution of the inflammatory infiltrates. This process is then followed by repair to the airways, so that after 7 days the lungs appear to be relatively healthy (6, 14, 19).

Previous studies demonstrate that T lymphocytes are important in the initial innate resistance to overwhelming nocardial invasion and growth within the murine lung (18). It is thought that these T cells are responsive to nocardial-induced epithelial damage and that they play a critical role in both PMN recruitment and localized epithelial repair (16, 18). The purpose of the study reported below is to determine whether the number of T cells in the lungs is increased during the termination of nocardial growth and the resolution of lesions 5 to 7 days after pulmonary infection with N. asteroides GUH-2.

	MATERIALS AND METHODS

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Mice. Six- to 8-week old C57BL/6J wild-type and C57BL/6J-Tcrd ( T-cell deficient) female mice were purchased from Jackson Laboratories (Bar Harbor, Maine) and housed in pathogen-free conditioned animal rooms at the University of California at Davis. The mice were maintained on Purina lab chow and provided water ad libitum. All mice were maintained by the animal resource services (ARS) following standard and approved protocols. The ARS monitored sentinel mice for infectious agents, and none were reported during these studies.

N. asteroides infection. N. asteroides strain GUH-2 was grown to mid-log phase in brain heart infusion broth (BHI-b; Difco) at 37°C with mild agitation for 16 to 19 h prior to use. The culture was checked for purity by plating a small sample onto BHI agar, and the morphology of the bacteria was monitored microscopically. Then, the culture was centrifuged for 10 min at 50 × g to pellet bacterial aggregates (7). The concentration of bacteria remaining in the supernatant was measured by optical density at 580 nm using a spectrophotometer (Beckman DU 640). The inoculum was then adjusted with fresh BHI-b to approximately 6 × 10⁷ CFU/ml. Serial dilutions were plated on BHI agar to enumerate the actual number of CFU. Either female C57BL/6J or C57BL/6J-Tcrd mice were anesthetized by intraperitoneal injection with Nembutal (50 mg/kg of body weight). Fifty microliters of the nocardial suspension was placed onto the anterior nares, and the mice were permitted to aspirate the inoculum. Previous studies revealed that this procedure reproducibly delivered approximately 1 × 10⁶ to 3 × 10⁶ CFU/lung (6). Control mice from each group were intranasally administered BHI broth without bacteria.

Lung homogenization and determination of CFU. To assess the actual dosage of GUH-2 in the lung, animals were randomly selected from each group of mice that recovered from the infection process. Three hours after infection, the lungs were removed, placed in 3 ml of Hanks' balanced salt solution (Gibco), and homogenized with a tissue homogenizer (Polytron PT1200). This lung homogenate was serially diluted and plated onto BHI agar. The BHI agar plates were incubated at 37°C for 3 days, and then the colonies were counted. The average number of CFU/mouse was calculated (6, 7).

Pulmonary murine lymphocyte isolation. At 3, 5, and 7 days postinfection, lungs from groups of mice were intracardially perfused with cold phosphate-buffered saline (PBS) at pH 7.4. The perfused lungs were then removed and minced in RPMI (Difco) to a fine slurry. This slurry was resuspended in 5 ml of digestion medium, which consisted of RPMI, 10% fetal calf serum (FCS), 10 U of DNase I and 20 U of collagenase type VIII per ml, and 50 µM -mercaptoethanol (1) and incubated for 2 h at 37°C. Undisrupted tissue in the digestion medium was teased with a 20-gauge needle and filtered through a 40-µm nylon cell strainer. The cell filtrate was washed and centrifuged at 200 × g for 10 min. This pellet was then resuspended in RPMI with 10% FCS, carefully overlaid onto a 40 to 70% Percoll gradient, and centrifuged for 30 min at 200 × g. Pulmonary murine lymphocytes were collected from the interface of the 40 to 70% Percoll gradient (1).

Multiparameter flow cytometry. The red blood cells that were contaminating the lymphocyte preparations were lysed with ammonium chloride. Then, 10⁶ cells were washed and incubated with 10 ng of anti-CD16/32 F_c block (PharMingen, San Diego, Calif.) per ml at 4°C for 20 min to reduce nonspecific binding. These blocked cells were dually stained using anti-CD3 T-cell receptor (TCR) conjugated to fluorescein isothiocyanate (PharMingen) and anti- TCR conjugated to phycoerythrin (PharMingen) for 15 min at 4°C. These stained cells were analyzed with a FACscan cytometer (Becton Dickinson). The resulting dot plot data were analyzed using Cellquest software.

Histology. To visualize both nocardiae and inflammatory infiltrates, the lungs from wild-type and T-cell-deficient mice were perfused with 10% neutral buffered formalin in 0.1 M PBS and embedded in paraffin. Paraffin sections from these lungs dissected at 3, 5, and 7 days postinfection were stained with the Brown and Brenn modification of the Gram stain as well as with hematoxylin and eosin (H&E).

Statistical analysis. Statistical significance was performed with the unpaired Student t test. All calculations were computed with Sigmaplot for Apple Macintosh. Statistical significance was indicated at P < 0.05 (n = 3 per experiment, three experiments).

	RESULTS

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Histological response in murine lung after infection with N. asteroides GUH-2. H&E staining of infected lung tissue (Fig. 1) showed that GUH-2 induced an extensive inflammatory response characterized by an infiltration of PMNs in both bronchiolar and alveolar regions of both normal and T-cell-deficient mice. (Fig. 1A and B). At 5 days after infection, histology revealed a divergent inflammatory response between the two groups of animals. The T-lymphocyte-deficient mice continued to show a strong PMN response, whereas the normal (wild type) mice appeared to be improving, with decreased inflammation (Fig. 1C and D). Seven days after infection, the lungs of wild-type mice appeared relatively normal, with only a few small regions of inflammation; in contrast, many centrally located foci of relatively large cellular infiltrates remained in the knockout mice (Fig. 1E and F). Thus, the level of infiltration and resolution of PMNs in the lungs of infected mice, as a function of time, was strongly impaired in the TCR-deficient (knockout) mice (Fig. 1 and 2).

View larger version (165K): [in this window] [in a new window]   FIG. 1.   Histology of the murine lungs (H&E stain) in normal and T-cell-deficient mice following intranasal infection with a nonlethal dose of log-phase cells of N. asteroides GUH-2. (A) T-cell knockout mice at 3 days after infection; (B) C57BL/6J wild-type mice at 3 days after infection; (C) T-cell knockout mice at 5 days after infection; (D) C57BL/6J wild-type mice at 5 days after infection; (E) T-cell knockout mice at 7 days after infection; (F) C57BL/6J wild-type mice at 7 days after infection. Original magnification, ×100. Bars, approximately 50 µm.

View larger version (136K): [in this window] [in a new window]   FIG. 2.   High-magnification view of lesion shown in Fig. 1E (bar, 10 µm). Note that lungs from TCR-deficient mice at 7 days postinfection contain foci of cellular infiltration, composed predominantly of PMNs.

Nocardial clearance delayed in TCR-deficient mice. To monitor nocardial clearance, Gram-stained tissues from wild-type and TCR-deficient mice were examined (Fig. 3). The divergent nature observed in histological examination was again paralleled in nocardial clearance. At 3 days, nocardial growth characterized by gram-positive filaments was unchecked in both murine strains (Fig. 3A and B). However, as observed with the PMN stabilization and resolution, the difference in nocardial clearance became apparent at five days. Gram-positive filaments were replaced in wild-type and TCR-deficient mice with gram-positive spherical bodies or L-phase variants of GUH-2 (Fig. 3C and D). These variants were more numerous throughout the lungs of knockout mice than those of their wild-type counterparts. At 7 days, lungs of wild-type mice appeared to be GUH-2 free (Fig. 3E), but gram-positive spherical bodies and nocardial filaments were observed in T-cell-deficient mice (Fig. 3F). These data indicated a delayed nocardial clearance in knockout mice.

View larger version (148K): [in this window] [in a new window]   FIG. 3.   Gram stain of N. asteroides GUH-2 in the lungs of normal C57BL/6J mice (B, D, and E) at 3, 5, and 7 days after infection compared to lungs from T-cell-deficient mice (A, C, and E) at the same time points. Arrowheads indicate gram-positive nocardial cells.

Increased T lymphocytes in nocardia-infected murine lungs. To determine the T-lymphocyte responses in the lungs following a nonlethal infection with N. asteroides GUH-2, multiparameter flow cytometry was used. A dot plot presentation of the data from flow cytometry showed a definite increase in T cells at 5 days after infection in the wild-type C57BL/6J mice compared to the wild-type control mice mock inoculated with BHI broth (Fig. 4H). The mice challenged with GUH-2 had a T-cell response that was first measurable at 3 days, peaked at 5 days, and remained elevated at 7 days after infection (Fig. 4G, H, and I). Also, BHI-b induced a slight but transient response in the lung that was probably due to nonspecific irritation in the lungs (Fig. 4D, E, and F). As expected, all T-cell knockout mice (negative control) failed to show any response in these assays. The use of TCR-deficient mice infected with GUH-2 confirmed the specificity of the TCR antibody and showed that the data were not artifactual, since the T cells detected in the knockout mice were insignificant at all time periods (Fig. 4A, B, and C). Thus, the lungs of mice infected with N. asteroides GUH-2 had increased numbers of T cells that coincided with resolution of the inflammatory response, clearance of bacteria, and repair of damaged lung architecture (compare Fig. 1, 3, and 4).

View larger version (58K): [in this window] [in a new window]   FIG. 4.   Representative dot plots obtained by FACscan flow cytometry of murine pulmonary T cells after intranasal instillation of log-phase cells of N. asteroides GUH-2 at 3, 5, and 7 days. (A, B, and C) Cells from T-cell knockout mice at 3, 5, and 7 days, respectively; (D, E, and F) cells from sham-inoculated control mice without bacteria at 3, 5, and 7 days, respectively; (G, H, and I) cells from wild-type mice infected with GUH-2 at 3, 5, and 7 days, respectively. Five thousand CD3+ cells per sample were analyzed. The test groups included C57BL/6J-Tcrd ( T-cell knockout mice) inoculated with GUH-2 in BHI broth, C57BL/6J wild-type mice inoculated with BHI broth alone, and C57BL/6J wild-type mice inoculated with GUH-2 in BHI broth. Numbers in each panel represent relative number of cells per quadrant compared to total cells. FITC, fluorescein isothiocyanate; PE, phycoerythrin; gd, TCR.

View larger version (17K): [in this window] [in a new window]   FIG. 5.   Relative abundance of T cells in murine lungs after intranasal inoculation of N. asteroides GUH-2. A summary of T-cell responses, expressed as percent T cells in the CD3+ population following intranasal infection with GUH-2 at 3, 5, and 7 days. The data points represent the mean ± standard error for each subgroup of three at each time point. Experiments were repeated two more times with similar results. The test groups included C57BL/6J-Tcrd ( T-cell knockout mice) inoculated with GUH-2 in BHI broth, C57BL/6J wild-type mice inoculated with BHI broth alone, and C57BL/6J wild-type mice inoculated with GUH-2 in BHI broth.

	DISCUSSION

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Murine responses to pulmonary infection by N. asteroides appear to depend on the inoculum, the time after infection, the presence of functional T lymphocytes, and an adequate PMN inflammatory response (6, 7, 13-15). With an intranasal inoculum dose slightly less than the 50% lethal dose (LD₅₀) for wild-type mice, T cells are critical to the initial innate resistance to massive and lethal pulmonary invasion by log-phase cells of N. asteroides GUH-2 (18). Thus, when equally challenged with a nearly LD₅₀ dose of log-phase GUH-2, T-cell knockout mice appear to be more deficient at PMN recruitment than their intact, wild-type littermates (16, 18). The lack of PMN recruitment renders T-cell-deficient mice more susceptible to GUH-2 than the controls (16). In the research described here, we did not observe the previously reported PMN deficiency in mice lacking T cells. However, it should be noted that our research utilized an inoculum dose threefold lower than that reported by King et al. (18). In these studies, we showed that recruitment of PMNs was not totally blocked, but delayed. In the studies reported above, we observed no mortality in nocardia-infected T-cell knockout mice using the lower inoculum dose (referred to as a nonlethal dose). We believe that these apparent differences may be related more to the initial severity of epithelial damage in the bronchioles caused by the higher inoculum dose than to a direct, protective role of the T cells per se (16, 18).

Studies have shown that T cells express mRNA for chemokines such as MIP-1, MIP-1, RANTES, and lymphotactin (10) and that neutrophil-dependent clearance of nocardiae requires CXC chemokines (19). These observations support the suggestion that T cells are responsible for the recruitment of inflammatory cells after epithelial damage (16). Therefore, even a slight delay in PMN recruitment to the site of nocardial invasion should have a profound influence on the outcome, since many investigators have established a critical role for PMNs in resistance to pulmonary nocardiosis (3, 5, 13-15, 19).

The data presented above demonstrated that the T-cell response in the lung was time dependent. Not only did T cells play an important role in initiating PMN recruitment, they also appeared to be involved in the resolution of the PMN response after infection. Following a nonlethal intranasal infection, there was a time-dependent peak in increased numbers of T cells in the lungs. At 3 days postinfection, the relative percentage of T cells had not increased much above basal levels. However, this ratio was altered dramatically 5 days after infection, when the T cells represented about 50% of the CD3-positive cells in the lungs. In contrast, the numbers of T cells remained at less than 8% of the total CD3-positive cells in sham-inoculated control mice. Histopathology of the pulmonary responses revealed a divergence in PMNs in lesions of mice with T cells compared to knockout mice lacking T cells. This dichotomy was most pronounced at 7 days after infection. At 5 to 7 days, there was a dramatic accumulation of T cells in the lungs of nocardia-infected wild-type mice concomitant with both the appearance and disappearance of neutrophils. These observations implied that the T lymphocytes expedited both the onset of the inflammatory response and its attenuation in order to prevent further tissue damage after the infection had been contained.

Numerous bacterial models that document T-lymphocyte participation exhibit variations on this theme (16). For example, in the Mycobacterium tuberculosis model, T cells regulate cellular trafficking, promote lymphocyte and monocyte infiltration, and limit inflammatory cells that may cause additional tissue damage (12). Furthermore, investigators utilizing the Listeria monocytogenes model concluded that T cells controlled the host inflammatory response to prevent excessive tissue damage (11, 17). The data presented above support the general hypothesis that T cells play a critical role in modulating host PMN responses, repairing epithelial damage and serving as sentinels at mucosal barriers (16, 17).